Optical measurement apparatus and electrode pair thereof

ABSTRACT

In an apparatus in which an electrode pair formed of two electrodes including multiple mutually parallel linear electrode pieces is provided in a container for storing particles dispersed movably in a medium to form an spatially regularly arranged electric field, the particles are migrated in the container due to the formation of the electric field by the application of a voltage to the electrode pair to generate a diffraction grating resulting from density distribution of the particles, diffracted light obtained by applying light to the diffraction grating is measured, and a particle size analysis or the like is performed from the temporal change in the diffracted light in the free diffusion process of the particles by stoppage or modulation of the application of the voltage, by making width L of the electrode pieces of the electrode pair and a space distance S between the electrode pieces as follows:
 
 L /( L+S )≦0.3,
         steep attenuation of the diffracted light intensity is not generated at the initial stage of the diffusion of the particles to accurately measure the particle size distribution.

TECHNICAL FIELD

The present invention relates to an apparatus for optically measuringinformation about the diffusion of particles in a sample having theparticles dispersed movably in a medium and information about theparticle size, the viscosity of the medium, and/or the migration of theparticles and an electrode pair used in the apparatus, in particular, toan apparatus for forming periodic electric field distribution in asample having particles dispersed movably in a medium to generate adiffraction grating resulting from density distribution of the particlesand measuring information about the diffusion of the particles andinformation about the particle size, the viscosity of the medium, and/orthe migration acting on the particles from the temporal change in thediffracted light from the diffraction grating and an electrode pair usedin the apparatus to form electric field distribution in the sample.

BACKGROUND ART

Particles with a diameter of 100 nm or less are generally callednanoparticles, and are just beginning to be used in various fieldsbecause they have properties different from those of general bulkmaterials of even the same material. Various methods such as the laserdiffraction/scattering method have been known as the method formeasuring the particle size. Among them, methods based on the so-calleddynamic scattering method (the photon correlation method) have beenemployed mainly for nanoparticles with a diameter of 100 nm or less(refer to Patent Literatures 1 and 2, for example).

The dynamic scattering method utilizes the Brownian motion of theparticles. According to the method, particles performing a Brownianmotion in a medium are exposed to a light beam, the intensity ofscattered light from the particles is measured at a predeterminedposition, the fluctuation of the scattered light intensity caused by theBrownian motion of the particles, that is, the temporal change in thescattered light is captured, and the particle size distribution of theparticles to be measured is calculated by utilizing the fact thatparticles each perform a Brownian motion with the intensity according toits particle size.

However, in the dynamic scattering method (the photon correlationmethod) in which the fluctuation of scattered light from the particlesis measured, the fluctuation of the scattered light to be measured isimperceptible in the case of microparticles because the intensity of thescattered light from the microparticles is proportional to the fifth tosixth power of the particle size. Due to its principle, the problems ofpoor measurement sensitivity as well as poor S/N cannot be avoided.

As a powerful approach for solving such unavoidable problems in thedynamic scattering method, there has been proposed a method and anapparatus for electrophoresing particles dispersed movably in a mediumby applying a spatially periodic electric field to the particles,generating a quasi-diffraction grating by making the particles have aspatially periodic alteration in concentration, in this state, detectingdiffracted light obtained by exposing the particles to a parallel lightflux such as a laser beam, and calculating the diffusion coefficient andthe size of the particles from the temporal change in the diffractedlight after stopping the application of the electric field (refer toPatent Literature 3).

The method and the apparatus proposed above utilize dielectrophoresis orelectrophoresis of the particles in the medium, and utilizes the factthat, from the state where a diffraction grating resulting fromconcentration distribution (density distribution) of the particles isgenerated by applying an electric field, the annihilation process of thediffraction grating by stopping the application of the electric fielddepends on the diffusion coefficient of the particles. The diffusioncoefficient and therefore the size of the particles can be calculatedfrom the time required for dissipation of diffracted light from thediffraction grating resulting from the density distribution of theparticles.

In the measurement method and the apparatus described above, thediffraction grating resulting from the density distribution of theparticles is formed in the vicinity of an electrode pair for applyingthe electric field to the sample to induce the dielectrophoresis of theparticles. An electrode pattern with which diffracted light from theelectrode pair and the diffracted light from the diffraction gratingresulting from the density distribution of the particles can beseparately measured is also proposed (refer to Patent Literature 4, forexample).

That is, each of electrodes constituting the electrode pair includesmultiple mutually parallel linear electrode pieces and a connection partelectrically connecting the respective electrode pieces to each other.Each electrode has a pattern that electrode piece ununiformly-arrangedareas including at least two linear electrode pieces arranged adjacentlyto each other, and electrode piece absent areas with no electrode piecearranged therein are formed alternately. The electrode pairs are formedby arranging the electrodes so that the electrode pieceununiformly-arranged areas of one electrode are positioned in theelectrode piece absent areas of the other electrode respectively, andthe electrode pieces are arranged in parallel with each other.

With the configuration above, when a voltage is applied to between theelectrode pieces, high-density areas of the particles are formed only ina part where the electrode pieces of one electrode are adjacent to theelectrode pieces of the other electrode. Thus, a grating pitch of thediffraction grating resulting from the density distribution of theparticles is larger than a pitch of the electrode pieces. Thereby, thediffracted light of the specific order from the diffraction gratingresulting from the density distribution of the particles, such as thediffracted light of the [2m+1]th order (m is an integer) in the casewhere two electrode pieces are ununiformly arranged in respectiveelectrodes, has the outgoing direction which is different from that ofthe diffracted light from the diffraction grating formed by theelectrode pieces. Thus, the diffracted light by the density distributionof the particles can be selectively detected.

In accordance with the method and the apparatus proposed above, theintensity of the diffracted light from the diffraction grating resultingfrom the concentration distribution of the particles is detected, andthus the intensity is greater than that of scattered light fromparticles obtained in the dynamic scattering method, thereby a moreintense signal is to be measured, resulting in a significant improvementin S/N and sensitivity relative to the dynamic scattering method.

The present inventors clarified that calculation for obtaininginformation about the diffusion coefficient and information about theparticle size and the like from the temporal change in the diffractedlight measured by the method based on the proposals above can beextremely simplified and also the information can be accurately obtained(refer to Non-patent Literature 1, for example).

That is, assuming that I represents the diffracted light intensity inthe annihilation process of the diffraction grating resulting from thedensity distribution of the particles, I₀ represents the starting valueof the diffracted light intensity (immediately after the start of theannihilation), D represents the diffusion coefficient of the particlesto be measured, and Λ represents the grating period, they areapproximated by the following expressions (1) and (2).

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{Expression}\mspace{14mu} 1} \right\rbrack & \; \\{I = {I_{0}{\exp\left( {{- 2}\;{Dq}^{2}t} \right)}}} & (1) \\\left\lbrack {{Mathematical}\mspace{14mu}{Expression}\mspace{14mu} 2} \right\rbrack & \; \\{q = \frac{2\;\pi}{\Lambda}} & (2)\end{matrix}$

The size “d” of the particles to be measured can be obtained from thefollowing Einstein-Stokes relational expression using such diffusioncoefficient D obtained from the measured value I of the diffracted lightintensity in the annihilation process of the diffraction grating. Theviscosity η of the medium can also be obtained by using particles whoseparticle size “d” is known.

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{Expression}\mspace{14mu} 3} \right\rbrack & \; \\{D = \frac{k_{B}T}{3\;\pi\;\eta\; d}} & (3)\end{matrix}$

In the expression (3), k_(B) is the Boltzmann constant, and T representsan absolute temperature.

Patent Literature 1: U.S. Pat. No. 5,094,532

Patent Literature 2: Japanese Patent Laid-Open Publication No.2001-159595

Patent Literature 3: Japanese Patent Laid-Open Publication No.2006-84207

Patent Literature 4: WO/2007/010639

Non Patent Literature 1: “Nanoparticle size analysis with relaxation ofinduced grating by dielectrophoresis” Yukihisa Wada, Shinichro Totoki,Masayuki Watanabe, Naoji Moriya, Yoshio Tsunazawa, and Haruo Shimaoka,OPTICS EXPRESS, 12 Jun. 2006/vol. 14, No. 12, pp 5755-5764

DISCLOSURE OF THE INVENTION

The Problems to be Solved by the Invention

With the electrode pair pattern described in Patent Literature 4 above,the diffracted light from the electrode pair and the diffracted lightfrom the diffraction grating resulting from the density distribution ofthe particles can be surely separated. However, the sizes of theelectrodes, specifically width L of the linear electrode pieces and aspace distance S between the adjacent electrode pieces are not at allknown. In the figures, the sizes L and S are shown as substantiallysimilar sizes, that is, L/(L+S)=0.5.

In Non-patent Literature 1, the sizes of the electrodes are clearlydescribed as L/(L+S)=0.5. By using such an electrode pair pattern, avoltage is applied to between the electrodes of the electrode pair tocause the migration of the particles and the diffraction gratingresulting from the density distribution of the particles is generated.After that, when the application of the voltage is stopped to freelydiffuse the particles and the temporal change in the diffracted lightintensity in the process of annihilating the diffraction grating ismeasured, steep attenuation of the diffracted light intensity isslightly observed at the initial stage of the diffusion (this is clearlyseen in measurement results of the particles with a diameter of 5 nmshown in FIG. 7 of Non-patent Literature 1). As discussed in Non-patentLiterature 1, such steep attenuation of the diffracted light intensityat the initial stage is assumed to be from the expressions (10) and (11)thereof or the like even in the measurement of the particles with asingle diameter. Assuming that the analytical expressions are right withsufficient accuracy, it is predicted that the initial attenuation abovewould differently act according to a complex refraction index of theparticles. It is thought that steep attenuation of the diffracted lightintensity is observed at the initial stage with the absorptioncoefficient of zero as shown in FIG. 5 of Non-patent Literature 1.

Meanwhile, the same phenomenon is also generated in the case whereparticles with a smaller diameter exist in particles with a diametercalculated from the attenuation of the diffracted light intensityobserved at the intermediate stage of the diffusion. Therefore, when theparticles have particle size distribution, it is difficult to perform anaccurate analysis. FIG. 22 shows a measurement example regarding topolystyrene particles with a diameter of 60 nm. In FIG. 22, thehorizontal axis indicates the time, the vertical axis indicates thediffracted light intensity by an exponential function, a solid lineindicates an actually measured value, and a broken line indicates acalculated value by the theory approximation expression. FIG. 22 clearlyshows that the actually measured value is deviated from the valuecalculated by using the theory approximation expression for about alittle more than 2 seconds at the initial stage of the diffusion. Thisindicates that the particle size cannot be analyzed by using the theoryapproximation expression for about 3 seconds at the initial stage of thediffusion, in the case where a sample including particles with themaximum diameter of 60 nm is measured for example.

Next, FIG. 23 is a graph showing, in a superimposed state, an actuallymeasured value (a solid line) of the temporal change in the diffractedlight intensity in the case where the particles with a diameter of 10 nmare included by an equal amount to the particles with a diameter of 60nm, and a calculated value (a broken line) obtained by the theoryapproximation expression in the case where the attenuation rate of thediffracted light after 3 seconds passes from the start of the diffusionis substantially similar with the measured values, that is, a rate of0.49 of the particles with a diameter of 60 nm and a rate of 0.16 of theparticles with a diameter of 20 nm are included. As shown in the figure,it is extremely difficult to discriminate the two different mixedsamples from the inclination of the attenuation of the diffracted lightafter about 3 seconds passes from the start of counting.

The present invention is achieved in consideration with such asituation, and an object thereof is to provide an optical measurementapparatus capable of accurately measuring particle size distributioneven for particles to be measured with arbitrary size distribution, andan electrode pair thereof.

Means for Solving the Problems

In order to solve the problems above, an optical measurement apparatusaccording to the present invention includes: a container for storing asample having particles dispersed movably in a medium; a power sourcefor generating an AC or DC voltage; an electrode pair for generating anelectric field having a space period in the container by application ofthe voltage from the power source; an irradiation optical system forapplying a parallel light flux to a diffraction grating resulting fromdensity distribution of the particles generated in the container by theapplication of the voltage; a detection optical system for detectingdiffracted light generated from the parallel light flux passing throughthe diffraction grating; voltage control means for controlling togenerate and annihilate the diffraction grating resulting from thedensity distribution of the particles in the container by theapplication of the voltage from the power source to the electrode pairand stoppage and modulation of the application of the voltage; and dataprocessing means for taking in outputs of the detection optical systemto execute the process of evaluating characteristics of the particlesand/or the medium. In the optical measurement apparatus, electrodesconstituting the electrode pair include multiple mutually parallellinear electrode pieces and a connection part electrically connectingthe respective electrode pieces to each other, respectively, electrodepiece ununiformly-arranged areas including at least two of the linearelectrode pieces arranged adjacently to each other, and electrode pieceabsent areas without the electrode pieces arranged therein arealternately formed in each of the electrodes, the electrodes arearranged so that the electrode piece ununiformly-arranged areas of oneof the electrodes are positioned in the electrode piece absent areas ofthe other electrode, and the electrode pieces of the electrodes are inparallel with each other at regular intervals, and a relationshipbetween width L of the electrode pieces of each of the electrodes and aspace distance S between the electrode pieces is as follows:L/(L+S)≦0.3.

The electrode pair according to the present invention is used in theoptical measurement apparatus above, and has the pattern and thedimensional ratio described above. The electrode pair is formed of avapor deposition film of a conductive body on a surface of a transparentflat plate.

The present invention is achieved as a result of earnestly examining thecause of a steep decrease in the diffracted light intensity at thebeginning of the start of the diffusion of the particles by the stoppageor the modulation of the application of the electric field aftergenerating the diffraction grating resulting from the particles.Hereinafter, an action based on the configuration of the presentinvention will be described with the process of reaching to theconfiguration.

Firstly, by numerically simulating the temporal change in the diffractedlight intensity based on the Fraunhofer diffraction theory, arelationship between an electrode shape and a profile of the attenuationof the diffracted light was simulated. FIG. 9 shows a profile ofparticle density (concentration) used in the simulation. In FIG. 9, thehorizontal axis indicates a position (the grating pitch direction) bythe unit of m, and the vertical axis indicates particle concentrationwith one serving as 100%. The amplitude of phase modulation and thetransmission amplitude by the particles are respectively set to be 0.01and 0.0002 proportional to the profile of the particle density. As shownin the figure, the profile of the particle density is such that width ofthe high-density areas of the particles is 5 μm, and a space between thehigh-density areas is 15 μm (the grating pitch is 20 μm).

FIG. 10 shows simulation results of the attenuation of the diffractedlight without the electrodes, that is, the temporal change in thediffracted light intensity when the particles are freely diffused afterthe generation of the diffraction grating resulting from the densitydistribution of the particles. The horizontal axis indicates the time,and the vertical axis indicates the diffracted light intensity which isdisplayed as logarithm and is a value normalized with using thediffracted light intensity immediately before the start of the diffusionas one. The calculation is performed with a diameter of the particles of60 nm, the medium of water, and a temperature of 25° C. The calculationis performed to obtain a ratio showing how the attenuation coefficientof the diffracted light is different from the following exponentialfunction which is the theory approximation expression:

[Expression 4 ]exp[−2q²Dt]  (4)

FIG. 11 shows results of the calculation. In FIG. 11, the horizontalaxis indicates the time, and the vertical axis indicates a coincidenceindex with the exponential function by the expression (4), which is asshown below:

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 5} \right\rbrack & \; \\\frac{{I\left( {t + {\delta\; t}} \right)} - {I(t)}}{{\exp\left\lbrack {{- 2}\; q^{2}D\left\{ {t + {\delta\; t}} \right\}} \right\rbrack} - {\exp\left\lbrack {{- 2}\; q^{2}{Dt}} \right\rbrack}} & (5)\end{matrix}$

FIG. 11 shows that the attenuation of the diffracted light almostcorresponds to the theory approximation expression over the entirecalculation time.

Next, the electrode pair is provided and then the calculation isperformed. At the time, the width of the multiple electrode piecesconstituting the electrode pair is set to be 5 μm, and the space betweenthe electrode pieces is set to be 5 μm. Then, the calculation of thediffracted light in the case where the high-density areas of the densitydistribution of the particles in FIG. 9 correspond to the space betweenthe electrodes is performed. This corresponds to a schematic profile ofparticle distribution in the case where the particles are collected dueto the electric migration to generate the diffraction grating by theelectrode pair configuration described in Patent Literature 4 above,that is, the electrode pair with the pattern with which the electrodepiece ununiformly-arranged areas of one electrode which include the twoelectrode pieces adjacently arranged are fitted into the electrode pieceabsent areas of the other electrode without electrode pieces arrangedtherein.

FIG. 12 shows the attenuation of the diffracted light, and FIG. 13 showsthe coincidence index with the exponential function serving as thetheory approximation expression. FIG. 13 clearly shows that by addingthe electrode pair with the shape and the size described above, theinclination is increased more than the exponential function predictedfrom the theory at the initial stage of the diffusion, that is, thediffracted light intensity is steeply attenuated at the initial stage ofthe diffusion.

FIGS. 12 and 13 are almost similar to FIGS. 10 and 11 except that aninfluence of the electrodes is taken into consideration or not.Basically, the figures show that steep attenuation at the initial stageof the diffusion is generated by the existence of the electrode pairwith the width of the electrode pieces of 5 μm, and the space betweenthe electrode pieces of 5 μm.

Next, in order to examine whether or not the existence of the electrodesitself is the cause of steep attenuation of the diffracted lightdescribed above, same profile of the particle density as in FIG. 9 isused for the calculation, but only the electrode pattern is changed,that is, the width of the electrode pieces is 1 μm and the space betweenthe electrode pieces is 9 μm. FIG. 14 is a graph showing thetransmission by the electrode pattern, FIG. 15 is a graph showing theattenuation of the diffracted light, and further FIG. 16 is a graphshowing the coincidence index with the exponential function serving asthe theory approximation expression described above. The figures clearlyshow that the existence of the electrodes itself is not the cause ofsteep attenuation described above.

However, in the case of the electrode pattern with the width of theelectrode pieces of 1 μm and the space between the electrode pieces of 9μm, the profile of the particle density is originally to be differentfrom the profile of FIG. 9, and the high-density areas of the particlesare to be formed in the entire space between the electrode pieces.

The profile of the particle density is changed to the profile of FIG.17, and the calculation is firstly performed for the case where theelectrodes do not exist. FIG. 18 shows the profile of the attenuation ofthe diffracted light, and FIG. 19 shows the coincidence index with theexponential function serving as the theory approximation expression.FIG. 19 shows that the attenuation of the diffracted light wellcorresponding to the exponential function of the theory approximationexpression can be obtained with the profile of the particle density ofFIG. 17.

Next, the electrode pattern shown in FIG. 14, that is, the electrodepattern with the width of the electrode pieces of 1 μm and the spacedistance between the electrode pieces of 9 μm is added to the profile ofthe particle density shown in FIG. 17 to perform the calculation of thediffracted light. FIG. 20 shows the profile of the attenuation of thediffracted light, and FIG. 21 shows the coincidence index with theexponential function serving as the theory approximation expression. Thefigures clearly show that the attenuation of the diffracted light wellcorresponds to the exponential function of the theory approximationexpression irrespective of the existence of the electrodes.

Thus, it is found that when the width of the electrode pieces isexpressed as L and the space between the electrode pieces is expressedas S in the electrode pair pattern for evoking the diffraction gratingresulting from the density distribution of the particles, there is arange where the attenuation of the diffracted light is regarded assubstantially the same as the exponential function when “L/(L+S)” isequal to a value from 0.1 to 0.5.

It should be noted that the finding above is only the investigationbased on the numerical calculation, and thus there is a possibility thatdifferent results may be obtained due to an error of the numericalcalculation or according to actual distribution of particle collection.Therefore, as will be described later, electrode pairs with variouswidths of electrode pieces and various space distances between theelectrode pieces were actually produced and experiments were performedby using actual samples having particles dispersed in mediums. Thereby,the simulation results above were examined to explore an electrode pairpattern capable of solving the problems. As a result, it was confirmedthat by making a relationship between width L of the electrode pieces ofelectrodes constituting the electrode pair and a space distance Sbetween the electrode pieces as follows:L/(L+S)≦0.3,

the attenuation rate of the diffracted light corresponds to the theoryapproximation expression.

By using the electrode pair with such a pattern, the particle sizedistribution analysis can be performed even for a sample in whichparticles having a plurality of size are mixed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of an embodiment of the presentinvention including a schematic diagram showing an optical arrangementand a block diagram showing an electric arrangement.

FIG. 2 is a diagram showing an electrode pair pattern in the embodimentof FIG. 1.

FIG. 3 is a graph showing an example of a voltage waveform applied to anelectrode pair and an example of the temporal change in the intensity ofdiffracted light from a diffraction grating resulting from densitydistribution of particles at the time of measurement in the embodimentof FIG. 1.

FIG. 4 is a graph showing results of experiments performed by usingvariously different width of the electrode pieces of the electrode pairand space distances between the electrode pieces in the embodiment ofFIG. 1, and the graph showing the attenuation coefficient ratio obtainedby calculating attenuation coefficients at multiple attenuation pointsrelative to an average attenuation coefficient Γ₀(=2q²D) at the latestage of attenuation of the diffracted light for the respectiveelectrode pairs.

FIG. 5 is a graph showing results of a particle size analysis with usingmeasured data of the diffracted light using the electrode pair with L/Sof 10 μm/10 μm among the electrode pairs shown in FIG. 4.

FIG. 6 is a graph showing results of the particle size analysis withusing measured data of the diffracted light using the electrode pairwith L/S of 1 μm/10 μm among the electrode pairs shown in FIG. 4.

FIG. 7 is a graph showing measurement results of the temporal change inthe diffracted light obtained when mixed particles having peaks inmultiple sizes are measured using the electrode pair with L/S of 1 μm/9μm.

FIG. 8 is a graph showing calculation results of particle sizedistribution obtained by performing the particle size analysis for themeasurement results of FIG. 7.

FIG. 9 is a graph showing a profile of particle density used innumerical simulation of the temporal change in the diffracted lightintensity.

FIG. 10 is a graph showing simulation results of the temporal change inthe diffracted light intensity when the particles are freely diffusedfrom the profile of the particle density shown in FIG. 9 in which noelectrodes exist.

FIG. 11 is a graph showing a coincidence index between the simulationresults of FIG. 10 and the theory approximation expression.

FIG. 12 is a graph showing simulation results of the temporal change inthe diffracted light intensity when the particles are freely diffusedfrom the profile of the particle density shown in FIG. 9 to which theelectrodes with the width of the electrode pieces of 5 μm and the spacebetween the electrode pieces of 5 μm are added.

FIG. 13 is a graph showing the coincidence index between the simulationresults of FIG. 12 and the theory approximation expression.

FIG. 14 is a graph showing the transmission of an electrode pattern ofelectrodes used when adding electrodes having the other pattern from thesimulation of FIG. 12.

FIG. 15 is a graph showing simulation results of the temporal change inthe diffracted light intensity when the particles are freely diffusedfrom the profile of the particle density shown in FIG. 9 to which theelectrodes of FIG. 14 are added.

FIG. 16 is a graph showing the coincidence index between the simulationresults of FIG. 15 and the theory approximation expression.

FIG. 17 is a graph showing a profile of the particle densitycorresponding to the electrode pattern of FIG. 14.

FIG. 18 is a graph showing simulation results of the temporal change inthe diffracted light intensity when the particles are freely diffusedfrom the profile of the particle density of FIG. 17 in which noelectrode pieces exist.

FIG. 19 is a graph showing the coincidence index between the simulationresults of FIG. 18 and the theory approximation expression.

FIG. 20 is a graph showing simulation results of the temporal change inthe diffracted light intensity when the particles are freely diffusedfrom the profile of the particle density shown in FIG. 17 to which theelectrode pattern shown in FIG. 14 is added.

FIG. 21 is a graph showing the coincidence index between the simulationresults of FIG. 20 and the theory approximation expression.

FIG. 22 is a graph showing a measurement result (a solid line) of thetemporal change in the diffracted light intensity regarding topolystyrene particles with a diameter of 60 nm by using a conventionalelectrode pair with width of electrode pieces of 5 μm and a spacedistance between the electrode pieces of 5 μm, and a calculation value(a broken line) by the theory approximation expression.

FIG. 23 is a graph showing an actually measured value (a solid line) ofthe temporal change in the diffracted light intensity in the case wherethe particles with a diameter of 10 nm is included by an equal amount tothe particles with a diameter of 60 nm, and a calculated value (a brokenline) obtained by the theory approximation expression in the case wherea rate of 0.49 of the particles with a diameter of 60 nm and a rate of0.16 of the particles with a diameter of 20 nm are included, theattenuation rate of the diffracted light after 3 seconds passes from thestart of the diffusion being substantially similar for both the values.

Reference Numerals  1 Sample cuvette  2 Electrode pair 21, 22 Electrodes21a, 22a Electrode pieces 21b, 22b Connection parts  3 Power source  4Irradiation optical system  5 Detection optical system  6 Dataprocessing and control section  P High-density areas of particles

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will hereinafter be describedwith reference to the accompanying drawings.

FIG. 1 shows an overall configuration of an optical measurementapparatus to which the present invention is applied, and FIG. 2 shows apattern example of an electrode pair 2 arranged in a sample cuvette 1.

The apparatus includes mainly: a sample cuvette 1 for storing a samplehaving particles dispersed movably in a medium, for example, a samplehaving particles dispersed in a liquid, or a sample composed of a gelhaving particles dispersed movably therein; an electrode power source 3for applying a voltage to an electrode pair 2 provided in the samplecuvette 1; an irradiation optical system 4 for applying light to thesample cuvette 1; a detection optical system 5 for measuring diffractedlight from a diffraction grating resulting from density distribution ofthe particles generated in the sample cuvette 1 through the applicationof a voltage to the electrode pair 2; and a data processing and controlsection 6 for collecting outputs from the detection optical system 5 toperform various analyses as well as for controlling the measuringoperations of the apparatus.

The sample cuvette 1 is composed of a transparent material such asglass, in which a plate-like member 20 also composed of a transparentmaterial is arranged fixedly, and the electrode pair 2 is formed on thesurface of the plate-like member 20.

As shown in FIG. 2, the electrode pair 2 includes comb-like electrodes21 and 22, and the electrodes 21 and 22 have multiple mutually parallellinear electrode pieces 21 a . . . 21 a and 22 a . . . 22 a andconnection parts 21 b and 22 b electrically connecting the respectiveelectrode pieces 21 a . . . 21 a and 22 a . . . 22 a to each other,respectively.

The electrodes 21 and 22 each have a shape in which electrode pieceununiformly-arranged areas including two linear electrode pieces 21 a or22 a arranged adjacently to each other and electrode piece absent areaswith no electrode piece arranged therein are formed alternately. Then,two electrode pieces 21 a or 22 a in each electrode pieceununiformly-arranged area of one electrode are fitted into eachelectrode piece absent area of the other and, as a whole, the electrodepieces 21 a and 22 a are arranged alternately two by two in parallelwith each other at regular intervals. The electrode pair 2 is formed byvapor deposition of a conductive body such as metal.

A relationship between width L of the electrode pieces 21 a and 22 a ofthe electrodes 21 and 22 and a space distance S between the adjacentelectrode pieces 21 a or 22 a is as follows:L/(L+S)≦0.3

When a voltage is applied from the power source 3 to the electrode pair2, electric field distribution is generated in the sample stored in thesample cuvette 1, and the particles in the sample are migrated due tothe electric field distribution as will be described hereinafter,thereby a diffraction grating resulting from the density distribution ofthe particles is generated. In this example, the power source 3 is an ACpower source, and the particles are moved by the dielectrophoreticforce.

The irradiation optical system 4 outputs substantially monochromaticlight shaped into a substantially parallel light flux, and the outputlight is applied to the electrode pair 2 in the sample cuvette 1. As alight source of the irradiation optical system 4, an element that emitsonly monochromic light such as a laser or an LED is easy to use.However, a continuous wavelength light source can also be used, if thelight thereof is made quasi-monochromic through a band pass filter, aspectrometer or the like. The spectrum bandwidth may be about tens nm orless, for example, within the visible wavelength range. In this example,the irradiation optical system 4 includes a laser 4 a and a collimationlens 4 b.

The detection optical system 5 is arranged in the outgoing direction of,for example, diffracted light of the first order diffracted by adiffraction grating resulting from the density distribution of theparticles in the sample cuvette 1 of the light from the irradiationoptical system 4. The detection optical system 5 includes, for example,a condenser lens 5 a, a pinhole 5 b, and a light detector 5 c. Thedetection optical system 5 measures the temporal change in the intensityof diffracted light from the diffraction grating resulting from thedensity distribution of the particles in the sample cuvette 1.

In the above-described arrangement, when an AC voltage from the powersource 3 is applied to between the electrodes 21 and 22 constituting theelectrode pair 2, electric field distribution according to the electrodepattern is formed in the sample within the sample cuvette 1, and densitydistribution of the particles is caused by dielectrophoresis based onthe electric field distribution. That is, in the electrode pair 2 shownin FIG. 2, high-density areas P of the particles are formed in a partwhere electrode pieces of reverse polarities are adjacent to each other,or in a part where the electrode pieces 21 a of one electrode 21 areadjacent to the electrode pieces 22 a of the other electrode 22 as shownin FIG. 2. The high-density areas P of the particles are formed in aspatially repeated manner at the pitch which is twice the arrangementpitch of the electrode pieces 21 a or 22 a, and in parallel with theelectrode pieces 21 a and 22 a. And a diffraction grating is formed bythe multiple high-density areas P of the particles. When the applicationof the voltage to the electrode pair 2 is stopped in the state where thediffraction grating exists, the particles start to be diffused andthereby the spatial density of the particles in the sample becomesuniform, and accordingly the diffraction grating resulting from thedensity distribution of the particles is annihilated in due course.

When the parallel light fluxes from the irradiation optical system 4 areapplied to the diffraction grating resulting from the densitydistribution of the particles, the light is diffracted by thediffraction grating. In the electrode pattern shown in FIG. 2, thediffraction grating resulting from the density distribution of theparticles has a grating pitch twice as large as that of a diffractiongrating formed by the electrode pieces 21 a and 22 a, so that thegrating constants are different between the diffraction gratings.Therefore, since the diffracted light from the diffraction gratingresulting from the density distribution of the particles and diffractedlight from the diffraction grating formed by the electrode pieces 21 aand 22 a appear in their respective different directions, only thediffracted light from the diffraction grating resulting from the densitydistribution of the particles can be detected by arranging the pinhole 5a and light detector 5 b at required positions.

The intensity of the thus detected diffracted light from the diffractiongrating resulting from the density distribution of the particlesgradually decreases during the process of annihilation of thediffraction grating. FIG. 3 is a graph showing an example of a voltagewaveform applied to the electrode pair 2 and an example of the temporalchange in the intensity of diffracted light from the diffraction gratingresulting from the density distribution of the particles. These examplesshow the case where a constant sinusoidal AC voltage V₀ is applied tothe electrode pair 2 to cause a dielectrophoretic force to operate onthe particles.

When the relationship between the width L of the electrode pieces 21 aand 22 a and the space distance S between the electrode pieces 21 a or22 a is set within the range as described above, steep attenuation ofthe diffracted light intensity is not observed at the initial stage ofthe diffusion of the particles. Thereby, the size distribution of theparticles can be accurately calculated.

In order to prove this, the following experiments were performed.“Electrode piece width L/space distance S” is used as the relationshipbetween the width L of the electrode pieces 21 a and 22 a of theelectrodes 21 and 22 constituting the electrode pair 2 and the spacedistance S between the adjacent electrode pieces 21 a or 22 a, andelectrode pairs having following values of “electrode piece widthL/space distance S between the electrode pieces” were actually produced:(a) 1 μm/10 μm; (b) 3 μm/10 μm; (c) 10 μm/10 μm; and (d) 3 μm/7 μm. Thevalue of L/(L+S) is about 0.09 in case of the electrode pair (a), about0.23 in case of the electrode pair (b), 0.50 in case of the electrodepair (c), and 0.30 in case of the electrode pair (d).

The electrode pairs above were used as the electrode pair 2 of theapparatus shown in FIG. 1, and a sample having polystyrene particleswith a diameter of 60 nm serving as particles to be measured dispersedin water was stored in the sample cuvette 1. After the diffractiongrating resulting from the particles was generated by the application ofa voltage to the electrode pair 2, the application of the voltage wasstopped to disperse the particles. The temporal change (the attenuation)in the diffracted light intensity in the dispersion process wasmeasured.

With regard to measurement results of the diffracted light intensityusing the electrode pairs above, attenuation coefficients at multipleattenuation points relative to an average attenuation coefficientΓ₀(=2q²D) at the late stage of the attenuation of the diffracted lightwere calculated, and the values were defined as the attenuationcoefficient ratio. Calculation results thereof are shown in the graph ofFIG. 4. In FIG. 4, the horizontal axis indicates the attenuation rate ofthe diffracted light by the unit of dB, and the vertical axis indicatesthe index number which is normalized with using the value of the averageattenuation coefficient which is average of the attenuation coefficientfrom the point that the diffracted light intensity is attenuated by −3dB (that is, one half) from the intensity immediately after thediffusion to the point attenuated by −10 dB (one tenth) therefrom, asone.

As shown in FIG. 4, in case of the electrode pair with “electrode piecewidth L/space distance S between the electrode pieces” of 10 μm/10 μm,the attenuation coefficient largely varies in a region where thediffracted light is attenuated by −5 dB from the initial intensity.Meanwhile, the attenuation coefficient is substantially constant in caseof the electrode pairs of 1 μm/10 μm and 3 μm/10 μm. In case of theelectrode pair of 3 μm/7 μm, it can be said that the attenuationcoefficient is substantially constant except slight variation at thevery initial stage. This demonstrates that in case of at leastL(L+S)≦0.3, the attenuation rate of the diffracted light corresponds tothe theory approximation expression. Therefore, when using an electrodepair with a pattern having such a size relationship, an accurateparticle size distribution analysis can be performed when measuringparticles having a plurality of sizes.

Next, for further confirming the above, the particle size analysis wasperformed with using the measurement results of the temporal change inthe diffracted light in the case where the electrode pairs with L/S of 1μm/10 μm and 10 μm/10 μm among the electrode pairs shown in FIG. 4 wereused. FIG. 5 shows results of the particle size analysis with usingmeasured data of the diffracted light using the electrode pair with L/Sof 10 μm/10 μm, and FIG. 6 shows results of the particle size analysiswith using measured data of the diffracted light using the electrodepair with L/S of 1 μm/10 μm. In the analyses, the particle size wasdivided into seventeen channels and the NNLS (Non Negative Least Square)method was used for the distribution analysis.

The particles to be measured are polystyrene particles with a diameterof 60 nm as described above. In the analytical results of the data usingthe electrode pair of 10 μm/10 μm, steep attenuation of the diffractedlight at the initial stage of the diffusion of the particles isreflected in the analysis result which looks as if small particlesexist, and as if large size distribution of the particles is suddenlybroken off in the distribution in the vicinity of the original particlesize. That is, the result shows physically improbable distribution.Meanwhile, in case of the electrode pair of 1 μm/10 μm, the result showsthe particle size distribution of a normal distribution, and namelyrational analytical results are obtained.

Next, with using the electrode pair with L/S of 1 μm/9 μm separatelyfrom the electrode pairs described above, experiments for measuringmixed particles having peaks in multiple size were performed. FIG. 7shows measurement results of the temporal change in the diffracted lightintensity in the diffusion process of the particles, and FIG. 8 showsresults of the particle size analysis which the measurement results weredivided into the particle size of seventeen channels by the NNLS methodas well as the analysis above.

From FIG. 8, the analytical result shows the particle size distributionhaving two peaks in particle size of 6 to 7 μm and 18 to 20 μm, andquasi-peaks of the particle size are not shown. Thus, an effect of theelectrode pair with L/(L+S)≦0.3 is obtained.

Industrial Applicability

According to the present invention, the generation of steep attenuationof the diffracted light at the initial stage of the diffusion of theparticles due to a structure of the measurement apparatus can besuppressed. As a result, even with the sample in which the measuredparticles having a plurality of size are mixed such as polydispersesystem particles, the particle size distribution can be accuratelymeasured.

1. An optical measurement apparatus, comprising: a container for storinga sample having particles dispersed movably in a medium; a power sourcefor generating an AC or DC voltage; an electrode pair for generating anelectric field having a space period in the container by application ofthe voltage from the power source; an irradiation optical system forapplying a parallel light flux to a diffraction grating resulting fromdensity distribution of the particles generated in the container by theapplication of the voltage; a detection optical system for detectingdiffracted light generated from the parallel light flux passing throughthe diffraction grating; voltage control means for controlling togenerate and annihilate the diffraction grating resulting from thedensity distribution of the particles in the container by theapplication of the voltage from the power source to the electrode pairand stoppage and modulation of the application of the voltage; and dataprocessing means for taking in outputs of the detection optical systemto execute the process of evaluating characteristics of the particlesand/or the medium, wherein electrodes constituting the electrode pairinclude multiple mutually parallel linear electrode pieces and aconnection part electrically connecting the respective electrode piecesto each other, respectively, electrode piece ununiformly-arranged areasincluding at least two of the linear electrode pieces arrangedadjacently to each other, and electrode piece absent areas without theelectrode pieces arranged therein are alternately formed in each of theelectrodes, the electrodes are arranged so that the electrode pieceununiformly-arranged areas of one of the electrodes are positioned inthe electrode piece absent areas of the other electrode, and theelectrode pieces of the electrodes are in parallel with each other atregular intervals, and a relationship between width L of the electrodepieces of each of the electrodes and a space distance S between theelectrode pieces is as follows:L/(L+S)≦0.3.
 2. The electrode pair used for the optical measurementapparatus according to claim 1, wherein the electrode pair is formed ofa vapor deposition film of a conductive body on a surface of atransparent flat plate.